Study co-author Fowler notes that in tracking individuals and social groups (through the Framingham Heart Study) over 30-years, the average size of each cluster of smokers was of similar size, but Fowler notes: “It’s just that there are fewer and fewer of these clusters as time goes on.”

The social contagion of quitting smoking can extend to people that the quitter didn’t know. For example if Anne quits smoking, and Anne is friends with Barb and Barb is friends with Clarissa, Clarissa’s chance of quitting increases by 30%, even if Anne and Clarissa don’t know each other.

Christakis notes that smokers have moved more to the periphery of social networks, where they often were more at the center of these social networks several decades ago. While this doesn’t say that teen non-smokers will necessarily be more popular, it does suggest over their lifespan that non-smoking is more likely to be associated with popularity than smoking.